Broad band directional couplers



y 1958 A. (5. FOX 2,834,944

BROAD BAND DIRECTIONAL COUPLERS Filed Oct. 29, 1954 5 Sheets-She et lsnow an N0 DEVICES 23 INVENTOR A. a. Fox

ATTORNEY May 13, 1958 A. G. FOX 2,834,944

BROAD BAND DIRECTIONAL COUPLERS Filed Oct. 29. 1954 FIG. 2A

5 Sheets-Sheet 2 F/G'4 cosa 43 Ba Ba E k: 451 2 3/ 9 E 4/ 33 v ll: 7/ 2COUPLING DISTANCE a /N RAO/ANS FIG. 414 6 7/" g 44 B1! 5 35 J= g cos av; SF 1 1 G 2 47 E t z k 46 B 0 k= s/- e COUPLING STRENGTH DISTANCEALONG COUPLING INTERVAL INVENTOR AGFOX ATTORNEY May 13, 1958 A. G. FOX2,834,944

- BROAD BAND DIRECTIONAL COUPLERS Fil'ed Oct. 29, 1954 5 Sheets-Sheet 5m/ VEN TOR A. 6. FOX

A T TORNE V May 13, 1958 A. G. FOX 2, 3

BROAD BAND DIRECTIONAL COUPLERS Filed 001:. 29, 1954 5 Sheets-Sheet 4COUPLING DISTANCE 9 INVENTOR 14.6. FOX

A TTORNE V May 13, 1958 A. 6. ox- 2,834,944

BROAD BAND DIRECTIONAL COUPLERS Filed Oct. 29, 1954 5 Sheets-Sheet 5COUPLING STRENGTH COUPLING DISTANCE 9 INVENTOR A. G./-' OX ATTORNEYUnited States Patent BROAD BAND DIRECTIONAL COUPLERS Arthur G. Fox,Rumson, N. J., assignor to Beil TelephoneiLaboratories, Incorporated,New York, N. Y., a corporation of New York Application October 29, 1954,Serial No. 465,579

23 Claims. (Cl. 333-) This invention relates to very high frequency ormicrowave electrical transmission systems and, more. particularly, tobroad band coupled. line systems for dividing electromagnetic waveenergy, such as directional couplers and coupled. line hybirds.

The directional coupler is a familiar component in high frequency and.microwave. transmission systems for which countless uses andapplications have been described in the. published art. In general, allpresently know-n directional couplers are. formed by a section of maintransmissionv line coupled to a section of auxiliary line. The couplingbetween the two sections is arranged so that an electromagnetic wavetraveling in a single direction in agiven mode of propagation along themain line induces-a. principal secondary wave, known as the forwardwave, traveling in a single direction in a given mode of propagationalongthe auxiliary line. Conversely, awave travelingin the oppositedirection in the main transmission line induces a principal secondarywave traveling in the opposite direction in the auxiliary line.

In most practical directional couplers there is also an induced orsecondary wave, known as the backward wave, even when the terminals arematch terminated, travelingin the opposite direction from the forwardwave. The forward and the backward waves are desirably greatly unequalin strength. Their relative strength is called the directivity of thecoupler. The relative strength of the desired induced forward wave inthe auxiliary line to the inducing wave in the main line is hereinafterreferred to as. the transfer ratio. The performance of the directionalcouplermay be described in terms of this directivityand transfer ratio.

The art is now familiar with a very large number of couplers ofdifferent forms each of which was developed inan effort to decrease thefrequency sensitivity of the coupler and to increase the operatingbandwidth over which a given value of directivity and a given transferratio are maintained. With the possible exception of certain veryspecialized ones of these couplers, a limiting value of frequencyselectivity is inherent in all of these couplers because they depend fortheir operation upon periodic reinforcement and cancellation ofcomponents from the main line in the auxiliary line.

It is therefore an object of the present invention to transfersubstantially only a forward traveling wave from one transmission lineinto another, which transferred wave bears aconstant ratio to theoriginal wave over anextended frequency band.

It is a further object to increase the operating bandwidth ot'icoupledline structures, such as directional couplers and hybrids.

In accordance with the present invention wave energy is launched andsupported in the. coupled transmission line: system: in a distribution;which will be. termed. a norma'l mode ofpropagation and-is transferredfrom Y is shifted from one guide into the other.

one line into. another by what is termed normal mode tapering.

Before proceeding further with a detailed consideration of the presentinvention, the new concept involving normal modes of coupledtransmission line propagation must be understood. This concept will notonly make it clear why directional couplers of the prior art types arefrequency selective but also why the couplers of the present inventionare independent of frequency. In a coupled wave guide system a normalmode is defined as that field distribution of the wave energy propagatedjointly in a pair of coupled guides that remains unchanged duringpropagation along a coupling region in which. all characteristics remainunchanged including the phase, coupling coefiicient, characteristicimpedance and the attenuation constant. In a system having two modes ofpropagation between which power transfer is to be effected, there aretwo normal modes into which wave energy propagating in one directionalong the pair of coupled guides can be resolved. Detailed considerationwill be given hereinafter to the nature of each. For the moment itshould be noted that one of these modes is designated the in phase mode,or the low phase velocity mode, since the two portions of the fieldintensity of the mode in each of the two coupled guides are in phase andpropagate at a lower velocity than would a conventional wave in eitherof the guides alone. The other normal mode is designated the out ofphase, or high phase velocity mode, since the two portions of itsintensity are out of phase and propagate at a higher velocity than woulda wave in either guide alone. Applying this concept to a directionalcoupler of the prior art types, it may be shown that when wave energy isdelivered to one of the wave guide terminals, both modes are equallyexcited. Since the two modes travel at unequal phase velocities alongthe length of the coupling interval, there will be subsequent regions inwhich the fields will be in phase in one guide and out of phase in theother and further regions in which this phase relation is reversed. Thischaracteristic is the one exploited in the prior art couplers to obtaintransfer of power from one guide into the other although the explanationgiven usually is taken from a different point of view. Since thedistance between the regions of periodic interference and reinforcementis a function of the difference in phase velocities or the guidewavelengths of the energy, it is apparent why the prior art couplers arefrequency sensitive.

In accordance with the present invention energy is excited in one normalmode only in a coupled system in which the parameters are chosen tosupport and propagate this single normal mode. The parameters are variedalong the coupler, however, so that the portion of the energy of thenormal mode carried in each guide In preferred embodiments of theinvention the parameter representing a phase constant difference betweenthe two coupled lines and the parameter representing the couplingcoeflicient therebetween are both tapered along the coupled system atinverse rates to each other. This results in a shift of energy travelingin the forward direction in one guide into energy traveling in theforward direction in the other guide that is theoretically unlimited inbandwidth. Since this shift is made over a length of severalwavelengths, there is little tendency for any backward waves to begenerated. Therefore the directivity of the coupler is high.

In other embodiments only the coupling coefficient is varied in a waythat produces a reversal in sign of the coupling. This also results in anormal mode tapered 2,834,944 r F f directional coupler of substantiallyincreased bandwidth over the couplers of the prior art.

In the copending application of J. S. Cook, Serial No. 465,578, filedOctober 29, 1954, there is disclosed and claimed a normal mode couplerof which only the phase velocity difference is varied.

The nature of the present invention, its various objects, features andadvantages will appear more fully upon consideration of the illustrativeembodiments shown in the accompanying drawings and the followingdetailed description thereof.

In the drawings: v

Fig. 1 is a perspective view of a first principal embodiment of theinvention showing a pair of wave guiding channels coupled with normalmode tapering produced by varying the relative phase constants and thecoupling coefiicients of the guides in accordance with the invention;

Figs. 2A through 2E, given by way of illustration. show the electricfield distribution in several pairs of coupled wave guides and alsorepresent the normal mode distribution in certain cross sections of theembodiment of Fig. 1;

Fig. 3 is a perspective view of a modification of the embodiment of Fig.1;

Figs. 4 and 4A show the relative phase constants and the couplingcoefiicients along an interval of coupling in accordance with preferredadjustments of the embodiment of Fig. 1;

Fig. 5 is a perspective view of a second modification of the embodimentof Fig. 1;

Fig. 6 is a perspective view of an embodiment in which the principles ofnormal mode tapering are applied to a coupled system of all-dielectricwave guides;

Fig. 7 is a perspective view of a normal mode directional coupler inwhich only the coupling coefiicient is varied along the couplinginterval;

Fig. 8 shows the relative phase constants and the coupling coeflicientalong the interval of coupling in accordance with a preferred adjustmentof the embodiment of Fig.7;

Fig. 9 is a perspective view of a modification of the embodiment of Fig.7; and

Fig. 10 shows the relative phase constants and the coupling coefficientalongthe interval of coupling in accordance with a preferred adjustmentof the embodiment of Fig. 9.

Referring more specifically to Fig. 1, an illustrative embodiment of anormal mode coupler is shown which produces a shift in energydistribution of the normal mode by tapering the wider cross sectionaldimensions of coupled, conductively bounded wave guides. As illustrated,this coupler comprises a section of rectangular wave guide of themetallic shielded type having a wide internal cross sectional dimensionwhich is substantially twice that of a conventional wave guide designedfor wave energy in a similar frequency band, being therefore slightlylarger than one free space wavelength of the energy to be conducted. Thenarrow cross sectional dimension is substantially one quarter of thewide dimension being equal to the narrow dimension of a conventionalguide.

Extending longitudinally along the length of section 20 is a conductivedivider 16-17-18 which divides guide 20 into two wave guiding channels21 and 22. Divider 161718 is serpentine in form with the portion 17starting in substantially the center of the far end of guide 20 andtapering to the right so that in the cross section aa the wide dimensionof channel 21 is substantially greater than the wide dimension ofchannel 22. Portion 18 tapers then toward the left until in section c-cthe wide cross sectional dimensions of channels 21 and 22 are equal. Thetaper then continues toward the left until in section e-e channel 21 hasa dimension substantially equal to the dimension of channel 22 insection A longitudinal distance of several wavelengths is includedbetwen sections aa and ee. Portion 16 tapers from its positionin'section e-e to substantially the center of guide 20 at the near end.

Channels 21 and 22 are coupled between the sections a-a and ee by adivided aperture 27 in portion 18 of divider 16-1718. Divided aperture27 has a transverse dimension which is zero, in section aa,,tapers to amaximum in section cc, and decreases again'to' zero in section e-e. Itis divided by a plurality of parallel wires 28 in accordance with theteachings of applicants copending application, Serial No. 236,556, filedJuly 15, 1951. Suitable coupling may also be provided by a plurality ofdiscrete apertures of tapered sizes spaced relatively close together inportion 18. The near ends of channels 21 and 22 are adapted to beconnected by conventional wave guide circuits to broad band devices 25and 23, respectively, while the other ends thereof are connected toloads 24 and 26, respectively.

The operation of the coupler employing normal mode tapering thus fardescribed from a structural standpoint will more clearly be understoodfrom the consideration of Figs. 2A through 2E which follows. It willfirst be assumed that these figures represent separate pairs of coupledwave guides having relatively different wide cross sectional dimensions.After the normal mode field patterns in such guides have been consideredit will be shown that these figures may represent the field distributionin the several cross sections aa through e-e of Fig. 1.

Referring to Figs. 2A through 2B, the electric field pattern of the inphase normal mode referred to above is shown as it would be supported inthe cross section of several conductively bounded transmission pathseach comprising two rectangular guides 11 and 12. In each figuresuitable coupling means is represented schematically by 13 in theconductive boundary between the contiguous narrow walls of guides 11 and12. Referring more specifically to Fig. 2C, the normal mode is shown inthe two guides 11 and 12 when the guides have equal wide dimensions andtherefore equal phase constants B and 3 (these parameters beingproportional to the wider dimension of the guides, for example). Thetotal field pattern of the mode comprises two portions of electricintensity on opposite sides of the conductive boundary 13 that aremaximum at the same instant of time and are in the same direction. Notethat each portion of the electric intensity forms a sine curvedistribution of less than a full half wave pattern since the twoportions are merged by the strong coupling to form the complete patterndefined as the normal mode. At the position of merger, i. e., at thecoupling means 13, both sine curves have equal amplitudes that aregreater than zero and in the same sense. Therefore the strength of thecoupling means 13 is represented by the distance k. The dottedprojections 14 and 15 of each sine curve are shown to illustrate theeffective transverse wavelength of the mode designated on Fig. 2C asThis mode is the low phase velocity one of the two normal modes sinceits transverse wavelength is greater than the transverse wavelength of awave that would normally be supported in an uncoupled guide of crosssectional dimensions equal to those of either guide 11 or guide 12. Sucha wave would propagate jointly along guides 11 and 12 without change inthe field distribution so long as the coupling and phase constants ofthe guides remained constant.

Now, in Fig. 2B guides 11 and 12 are modified so that ,8 is greater than5 and the coupling factor k is less than the factor k of Fig. 2C. Thetwo portions of the normal mode will no longer have equal amplitudes.Rather the distribution will be similar to that shown in Fig. 2B

with the amplitude of E in guide 11 substantially greater than theamplitude B in guide 12. Obviously,

there is a unique relationship between the difference fi -13 and thecoupling coeflicient k thatwill -support a given voltage distributionsince the transverse wave length of the two portions ofthe normal modemust always be equal in a given cross section. Conversely, for a givenphase constant difference between the guides and a given couplingcoefficient there is a given distribution of the normal mode energybetween the two guides.

In accordance with the present invention, therefore, energy is excitedin a coupled system in one normal mode only. While maintaining a givenrelationship to be de fined hereinafter between the phase constantdifference between the guides and the coupling-coefficient therebetweenso that the energy remains only inthe original normal mode, theseparameters are .varied smoothly and gradually to shift the energy of thenormal mode carried by one guide into the other.

A typical shift of this type may be seen by means of the sequence ofFigs. 2A through 2E. Starting with Fig. 2C, assume that wave energy inthe distribution discussed above has been excited by some means withequal amplitudes E and E in the two guides. Now, the parameters ofthe-two guides are varied smoothly until the cross section becomes thatrepresented by Fig. 2D, for which [i is much. smaller than [3 and k hasalso been decreased. The pattern of the normal mode will therefore shiftwith the amplitude E becoming smaller than the amplitude E If thedifference between the phase constants 5 and [3 is further increased andk is decreased to zero, as shown in Fig. 2E, all energy will be shiftedinto the right hand portion of the field pattern and the amplitude inthe left hand portion will be zero. A similar shift can be produced intothe left hand portion of the field pattern by decreasing 13 andincreasing ,8 as shown in the cross section sequence of Figs. 2B and 2A.

Since the shift action described above is reciprocal, Figs. 2A through2E may now be used to describe the operation of the coupler of Fig. l byallowing Fig. 2A to represent the cross section of guides 21 and 22 atthe section designated aa on Fig. l and Figs. 23 through 2E to representthe subsequent sections designated bb through e-e, respectively, with'the cross sections of guides 21 and 22 tapering smoothly between thesepositions. band device 23, serving as a source, to the forward terminalof guide 22, the field pattern of the wave in guide 22 will spread asthe wave passes taper portion 16 until at the section e-e it will havethe distribution of Fig. 2E. At the section cc it will be dividedequally between guides 21 and 22 as shown by Fig. 2C. At section a=a itwill be completely transferred into guideZl as shown by Fig. 2A to bedelivered at load 24. This transition is independent of frequency sinceit is made with the energy always in only one normal mode, in this casethe low velocity mode, and does not dependupon a periodic interferenceand reinforcement between several components, Also, except for possiblereflections due to impedance discontinuities, no energy will be directedin the backward direction toward load 25 thereby producing in thecoupler a high degree of directivity,

When broad band device 25 serves as a source of signals, a similar shiftof energy will transfer all power from guide 21 into guide 22 fordelivery to load 26. Transmission in this direction, however, is made inthe out of phase normal mode or the high phase velocity mode notedabove. A typical field pattern for this mode would look similar to thoseshown in Figs. 2A through 2E except that the two portions" of electricalintensity on opposite sides of the conductive boundary would be out ofphase and the transverse wavelengthof the mode would be the sameas thetransverse wavelength of a wave that would be supported in an uncoupled.guide. As a result thetwo portions of a wave in the cross section ccthat result from energy beingappliedtoguide- Therefore, as wave energyis applied from broad d 21 are equal in amplitude but out of phase, andsuch energy will bedelivered' to load 26 out of phase with respecttoenergy launched originally in guide 22- and delivered to load 24.

The structure thus described constitutes a directional coupler capableof complete power transfer and high directivity over an extremely broadband. When energy is initially excited in the branch having the largerphase constant, itwill' be delivered in an in phase normal mode at theother end in the branch having the larger phase constant. When energy isinitially excited in the branch having the smaller phase constant, itwill be delivered. in an out--of phase. normal mode at the other end inthe branch having the smaller phase constant.

The coupler, however, is not limited to complete power transfers. If thecoupling of aperture 27 was gradually closed starting from section aa,any desired division of power could bemade between loads 24 and 26; Astructure'producing the particular division of equal power, whichisthetypical characteristic of a hybrid type structure, is illustrated by themodification of Fig. 3 in which aperture 30 between the sections cc ande-e is substantially one half of aperture 27, i. e., an aperture endingwith its point of maximum coupling at the section of equal crosssectional dimensions of guides 21 and 22. An additional aperture section31 beyond the section c-c isprovided inthestraight portion of divider 17to avoid a sharp impedance discontinuity at section c-c. Wave energyfrom device 23 will therefore be divided equally and inph'asebetweenloads 24 and 26 with none appearing at 25; Wave energy from device 25will be divided equally and out of phase between loads 24 and 26 withnone appearing at 23. Aperture section 31 actually plays no part in thecoupling, other than to match the impedance as noted above since beyondthe section cc energy is alreadyequally divided in the normal modedistribution;

Referring again to Figs. 2A through 2E, it should be apparent inview ofwhat is taught above that there are an unlimited number of sequences ofintermediate field distributionsthrough which the energy may be shiftedin transition between the distributions of Fig. 2A and Fig. 2C orbetween Fig. 2C and Fig. 2E. It may be shown thatthe distance requiredto shift a given portion of the energy from one guide into the other isinversely proportional to the quantity in which 6 is equal to one halfthe difference in the phase constants [i and 5 of the guides and inwhich k is the coupling coeificient. In order to keep the coupler short,it is desirable then that this quantity be as large as possible. Thelimiting values are determined in the cross 1 sections of Figs. 2A and2E on one hand when 6 is maximum and k is zero, and in the cross sectionof Fig. 2C

on-the other, when 6 is zero and k is maximum.

Between these extremes the quantities 5'and k are varied smoothly atsubstantially inverse rates to each other. As used herein and in theappended claims the term inverse means only that as one quantity isincreasing with distance along the coupling interval, the other isdecreasing with distance and not necessarily that they are reciprocal orvary at equal absolute rates. More particularly, according to apreferred form of the invention, 8 in the sections of Figs. 2A and 2Eshould be substantially equal to k in the section of Fig. 2C so that thequantity /6 +k in the three limiting cross sections is equal.Furthermore both 6 and k are varied between these sections so that \'/5+k as calculated for each finite intermediate cross section issubstantially constant from one section to the next. These proportionsappear to give optimum performance of the coupler for the shortestcoupling interval. This action may be explained on thetheory that whenthe quantity 5 is constant dielectric constant.

7 along the interval the characteristic impedance presented to thenormal mode along the interval is also constant so that no impedancediscontinuity is present to distort and reflect components of theshifting wave.

A simple distribution meeting all these qualifications is illustrated inFig. 4. The coupling coefiicient k as represented by the characteristic41 is varied in accordance with a sine function and the phase constants{3 and 13 of the guides as represented by the characteristics 42 and 43are varied inversely to each other according to the corresponding cosinefunction along the coupled interval. For such a distribution the phaseconstant difference 0 in Equations 2 and 3 be varied non-linearly withdistance along the coupling interval, and more specifically, to varymore rapidly in the center portion of thecoupling interval and moreslowly at the ends of the interval. Such a characteristic is shown inFig. 4A by curve 47 which forms an S shape when 0 is plotted againstdistance along the coupling interval and having the region or rapidvariation at the center of the interval. 'Curves 44 and 45 represent theresulting phase constant characteristics and curve 46 the resultingcoupling strength characteristic for a condition of complete powertransfer. These characteristics exhibit maximum changes of rate in thecenter, of the coupling interval and vary at rates approaching zero atthe ends of the interval. If such a distribution is foreshortened as inthe structure of Fig. 3 to produce a transfer that is less thancomplete, maximum variation of 0 must still occur in the center of thecoupling interval. The corresponding phase and coupling characteristicswill still vary at maximum rate in the center of the coupling intervalbut will not necessarily approach zero at the ends of the interval. In.either case it should be noted that the desirable relation maintainingEquation 1 equal 'to a constant along the coupling interval ispreserved.

In the case of either complete power transfer or less than completetransfer, the S-shaped distribution of 0 produces a coupler in which ashorter coupling interval is required for a given bandwith, orconversely, a wider bandwidth for a given coupling interval.

It should be noted that in the preceding discussion, reference to thephase constant or phase velocity refers to properties of a-wave in oneof the guides as perturbed by the presence of the coupling slot andother wave guide.

However, if the coupling is small these. quantities are very nearly thesame as those for an uncoupled wave guide .and may be calculated fromits cross sectional dimensions,

and its dielectric and permeability constants.

In the embodiment of Fig. 1 the phase constants of the 'guides aretapered by tapering the wider cross sectional dimensions of the guidingchannel. The phase constants of the guides may also be tapered bytapering their dielec-' tric and/or permeability. constants. Referringto Fig. 5, an illustrativeembodiment of the invention is shown in whichthe guides are loaded by tapered members of high Guides 50 and 51 arerectangular wave guides of uniform cross sectional dimensions which arelocated side by side to provide a contiguous or common wall 52. Adivided aperture 53 several wavelengths long is located in common wall52 to provide a tapered coupling of the type described above betweencross sections a-a and cc. Located in guides and 51 are identical tapermembers 54 and 55, the cross sections of which, and therefore themasses, increase from zero to a maximum and then decrease to zero.Members 54 and 55 are so oriented that at the cross section aa member 54has a minimum mass and member 55 has a maximum mass; at cross sectionb-b, members 54 and 55 have equal masses; and at cross section c-c,member 54 has a maximum mass and member 55 has a minimum mass. Taperportion 56 of member 55 and portion 57 of member 54 provide areflectionless transition between the unloaded portions of the guidesand the maximum dimensions of members 55 and 54, respectively. Numerousand varied other physical arrangements of dielectric or permeabilitymaterial will occur to those skilled in the art which would providesimilarly tapered phase constants for guides 50 and 51.

The principles of the present invention are by no means limited toshielded transmission lines, either wave guide or coaxial, but maylikewise be applied to other forms of electrical transmission lines suchas the lines employed in the all-dielectric wave couplers disclosed inmy copending application, Serial No. 274,413, filed March 1, 1952. Asthere disclosed, electromagnetic wave energy, when properly launchedupon a strip or rod of all-dielectric material without a conductiveshield, will be guided by the rod with a portion of the energy conductedin a field surrounding the rod. These strips may be made of polystyrene,polyethylene or Teflon, for example, to mention only several specificmaterials.

Referring to Fig. 6, an all-dielectric directional. coupler employingnormal mode tapering is shown. This coupler comprises a straight strip69 of all-dielectric wave guide of the type hereinbefore described and asmoothly curved portion of a strip 61 of similar material which archesinto proximate relation to a portion of guide 60. The transverse crosssections of both guides 60 and 61 at the position of center line 62 aresymmetrical and, more particularly, circular, as represented by thecross sectional showin s 63 and 64. On either side of center line 62,guides 69. and 61 are squashed or deformed into ovoid transverse crosssections having different perpendicular transverse cross sectionaldimensions. More particularly, the left hand end 65 of guide 60 isdeformed into an elliptical cross section having its longer major axisin a vertical position. The right hand end 66 of guide 60 is deformedinto an elliptical cross section having its longer major axishorizontal. The left hand end 67 of guide 61 is deformed into an ellipsehaving its longer major axis horizontal while the right hand end 68 ofguide 61 has its longer major axis vertical. Guides 60 and 61 may beheld in this relative position in numerous ways such as the oneillustrated inthe above mentioned copending application comprising ablock 69 of material having a low loss and a low dielectric constant andwhich is provided with suitable slots into which guides 61 and 60 may bepressed. Also, the several alternative physical orientations for the twoguides as disclosed in said copending application may be used.

Since a substantial amount of wave power is carried in the spacesurrounding each guide, when the guides are brought into proximatephysical relationship the fields carried by the guides interact toproduce electromagnetic coupling between the two dielectric paths. Theamplitude of this coupling is inversely proportional to the distancebetween the guides. Therefore the spacing between guides 60 and 61 issuitably chosen to produce a distributed and tapered coupling whichgradually decreases from maximum coupling at center line 62 to aninfinitesimal coupling at points where the guides are separated by alarger amount. This .coupling is taperedim accordance. with-the desiredcoupling characteristic whichmay bethe sine curve 41 as illustrated inFig. 4;

Since the phase velocity of wave energy conducted by guides 6th and 61is inversely proportional to the thickness of the rod measured parallelto the polarization of the electric vector of the energy, it is apparentthat guide 60 will have a maximum phase velocity for wave energypolarized horizontally in portion 65 and will decrease to a minimumphase velocity for wave energy polarized horizontally in portion 66. Therate ofdeformation of guide 60 between portions 65 and 66 may thereforebe selected to produce any desired taper of the phase constant includingthe cosine curve 42 illustrated hereinbefore with reference to Fig. 4.Similarly, guide 61 provides a minimum phase velocity for the waveenergy polarized horizontally in the portion 67 and a maximum phasevelocity for wave energy polarized horizontally in the portion 68. Thusa maximum difference in the phase velocities between guides 60) and 61is provided at the points of minimum coupling removed from. center line62. while equal phase velocities in the two guides are provided atthepoint of maximum coupling at center line. 62. This then is the desiredphase velocity and coupling characteristicrepresented by Fig. 4. In allrespects a shift. of wave energy of the normal mode carried eitherhorizontally or vertically by guides 60 and 61 will be exhibited whichis similar to the shift demonstrated hereinbefore withreference to Figs.2A and 2E. Therefore wave energy applied in a horizontal polarizationrepresentedby the vector E to guide 69 will appear at the end of guide61 in the polarization represented by vector E and wave energy of. thepolarization represented by E; applied to guide 60 will appear on guide61 in the polarization represented by the vector E Similar directionalcoupling action exists for vertically polarized waves.

The embodiments thus far described illustrate how both the phaseconstants and the coupling coefficients may be varied to obtain normalmode taperingin a directional coupler structure. When tapered accordingto the optimum relationships given, the bandwidth of the directivity andtransfer ratio of the coupler are limited substantially only by theinherent bandwidth of the wave guide components. In some applications itmay be inconvenient or impossible to vary both of these parametersfreely and also a bandwidth somewhat less than the maximum will besatisfactory. In these cases normal mode tapering may still be employedif the single parameter of coupling or phase constant is varied in aparticular way. Variation of the phase constant alone is the subjectmatter of the above copending application of J. S. Cook.

in Fig. 7 is shown an embodiment of a normal mode directional coupler inwhich only the coupling coefiicient is varied, and more specifically,varied along the coupling interval from zero to a positive maximum,reversing in sign to a negative maximum, and decreasing again to zero.As shown in Fig. 7, thecoupler comprises a first section 71 and a secondsection 72 of rectangular wave guide. One narrow wall of guide 72 isplaced contiguous to and centered upon a wide'wall of guide 71. The widedimension a of guide 71. is somewhat larger than the wide dimension [7of guide 72 giving to guide 72 a larger phase constant than guide 72 asshown by the characteristics 81 and 82 of Fig. 8.

Guides '71 and 72 are electromagnetically coupled over two intervals,each extending several wavelengths along the longitudinal length of theline by divided apertures 73 and 74, respectively. The near end ofaperture 73 commences on the longitudinal center line of guide 71 atwhich point the longitudinal magnetic field components in guide 71 arezero. Aperture 73 is a fraction of a wavelength in width so thatitis'unresponsive to transverse magnetic field components in guide 71.Thus the coupling coefiicient at this pointis zero. Aperture. 73 extendstoward one wide; wall of guide 72 .nndtso extends toward increasinglylarger valuesof longitudinal field in guide 71; Therefore the couplingcoefficient of aperture 73 is represented by. characteristic 83 of Fig.8. Aperture 74 commences at a point on the opposite side of thelongitudinal center line ofguide 71 that corresponds to the terminationpoint of aperture 73', and extends to the center line of guide 71. Sinceaperture 74 couples to longitudinal field components in guide 71 havingopposite signs from those coupled by aperture 73; aperture 7 produces acoupling coefiicient that is represented by the characteristic 84 ofFig. 8.

A non-mathematical explanation of why the structure thus described willproduce anormal mode transfer of energy from one guide into the othermay be given by analogy to the transfer illustrated in Figs. 2A through2E described above. If wave. energy is applied to the near end of guide71', the initial conditions are the same as those existing in Fig. 2E,i. e., energy is applied to the guide of larger phase constant at apoint of zero and subsequently increasing coupling factor. Therefore,one of the pairs of normal modes will be initially excited in thecoupled system, being the in phase or out of phase mode depending uponwhat relative polarizations in. the two guides are defined as being inphase. An initial difference in the phase constants ,Ba and 5b preventsinitial excitation of the other normal mode in the region of smallcoupling. As the coupling of aperture 73 increases, an increasinglylarger portion of the normal mode energy will shift into guide 72 untilat the end of aperture 73' in the center cross section of the guidesubstantially equal portions of the normal mode will be found in guides71 and '72, which is analogous to the distribution of Fig. 2C.

Now, assume for the purposes of explanation that the coupling between.guides 71' and 72 is decreased in the second coupling region to zeroalong the broken line characteristic 85 of Fig. 8. In such an assumedstructure all the energy would return to guide 71, that is, the processperformed in the first coupling region would be reversed in the secondcoupling region. The, operation of the assumed structure therefore failsto follow the complete transfer, sequence of Figs. 2C through 2A becausethere is no crossover inthe phase constants of the guides as wasprovidedby the characteristics of B and {3 of Fig. 4. However, thecoupler of Fig. 7 actually provides an equivalent crossover by reversingthe sense of the coupling coefficients produced by aperture 74 relativeto aperture 73 as shown by the characteristics 84 and 83 of Fig. 8,respectively. Thetransfer sequence is completed and all energy in thenormal mode will appear at the far end of guide 72.

Further demonstration, that this is true may be made as follows. Ifwave'energy applied at the near end of guide 71 produces normal modecomponents in the center cross section of the coupler that are definedas in phase in guides'7l and 72, thenenergy applied to the far end ofguide '71 produces out of phase components. Similarly, wave energyapplied to the near end of guide 72 produces out of phase components atthe center while energy applied to the far end of guide 72 produces inphase co 2"- ponents. These relative phases are obvious from anexamination of the field patternsof the waves taking into account thereversal of the coupling coefficient in the two portions. Since allsections of the coupler are reciprocal, the energy distribution producedby exciting the near end of guide 71 andthefar end of guide match, andcoupling is established therebetween. Likewise the energy distributionproduced by exciting the near end of guide 72 and the far end of guide71 match, and coupling is establishedtherebetween.

In the embodiment of Fig. 9 the. coupling regions are transposedrelative to their positions in the embodiment of Fig. 7 with aninteresting result. Thus in Fig. 9 guides 91. and 92 are located withtheir wider walls contiguous and their longitudinal axes skewso that anelongated di- 11 vided aperture 93 runs diagonally in the wide wall ofguide 91 from a point near one narrow wall to a point near the othernarrow wall, but runs in the wide wall of guide 92 near and parallel toone narrow wall. This produces the coupling coeflicient represented bythe characteristic 96 of Fig. 9 that runs from a positive maximum at oneend of the coupling region, through zero, and to a negative maximum atthe other end of the coupling region. The wider cross sectionaldimension a of guide 91 is larger than the wide dimension b of guide 92giving to the guides phase constants 8a and flb, respectively, asrepresented by the characteristics 97 and 98 of Fig. 10.

Note that the phase and coupling characteristics at the ends of theembodiment of Fig. 9 correspond to these characteristics at the centerof the embodiment of Fig. 7. Conversely, the phase and couplingcharacteristics at the ends of the embodiment of Fig. 7 correspond tothese characteristics at the center of the embodiment of Fig. 9.Therefore the energy distribution at the several corresponding crosssections will also correspond. From this the interesting characteristicof the embodiment of Fig. 9 may be understood. If equal portions of waveenergy are applied in phase to the near ends of guides 91 and 92, equalout of phase portions will be produced at the far ends of the guides.This structure is therefore useful in any one of the many applicationsin which it is necessary to produce a phase inversion in a one-halfportion of energy relative to the other half portion. The inserted 180degree phase shift is, however, independent of frequency since it hasall the broad band characteristics of the normal mode couplingstructure.

In all cases, it is to be understood that the above-describedarrangements are simply illustrative of a small number of the manypossible specific embodiments which can represent applications of theprinciples of the invention. Numerous and varied other arrangements canreadily be devised in accordance with these principles by those skilledin the art without departing from the spirit and scope of the invention.

What is claimed is:

l. A coupling device comprising a pair of electromagnetic wavetransmission lines, means for coupling said lines along a givenlongitudinal section of said lines, said coupling varying smoothlybetween substantially different maximum and minimum coupling amplitudesat different points along said section, said lines having phaseconstants, the difference between said phase constants varying smoothlyalong said section with a maximum difference at substantially the pointof minimum coupling and a minimum difierence at substantially the pointof maximum coupling.

2. A coupling device comprising a pair of electromagnetic wavetransmission lines, means for coupling said lines with a variation inthe coupling coeficient along a given longitudinal section of saidlines, said coupling coefficient being a function of the rate of voltagechange with distance along said section in each of said lines, saidlines each having phase constants that vary inversely to each otheralong said section with the difference between said constants varyinginversely to said coupling coefficient along said section.

3. The combination according to claim 2 wherein the maximum value ofsaid coupling coefficient along said section is substantially equal toone half of the maximum difference between said phase constants alongsaid interval.

4. The combination according to claim 2 wherein the quantity vim ascalculated at each point along said section is constant, in which 6 isone half of the difference between said phase constants at each pointand k is the coupling coefficient at said point.

5. The combination according to claim 2 wherein the ditference betweensaid phase constants varies proportionally as the cosine of the distancealong said section and wherein said coupling coefficient variesproportionally as the sine of the distance along said section.

6. Directional coupling apparatus .for electromagnetic wave energycomprising a pair of conductively bounded wave guides, means forcoupling said guides with a tapered variation in coupling strength alonga longitudinal interval of said guides, a pair of members of highdielectric constant material located one in each of said guides, themass of each of said members being tapered with distance along saidinterval at substantially dilferent rates to each other, the mass of oneof said members being tapered with distance along said interval atsubstantially an inverse rate to the strength of said tapered coupling.

7. Microwave coupling apparatus comprising first and second dielectricmembers, said members each being substantially symmetrical in transversecross section in a center portion thereof and being ovoid in crosssection on either side of said center portion, each of said membersbeing unsheathed whereby a portion of electromagnetic wave powerconveyed therealong is conveyed in a field surrounding said members,said members being in prox imate physical relationship in said centerportion to provide interaction between the fields surrounding saidstrips.

8. Apparatus according to claim 7 wherein the longest dimension of saidovoid cross section of one member on one side of said center portion issubstantially parallel to the longest dimension of said ovoid crosssection of the other member on the other side of said center portion.

9. Apparatus according to claim 7 wherein at least one of'said membershas a smoothly curved portion of its length that arches into proximaterelationship to said center portion of the other of said members.

10. A coupling device comprising a pair of electromagnetic Wavetransmission lines, said lines having different phase constants, meansfor coupling said lines with a coupling coefiicient that varies withdistance over two intervals each several wavelengths long, the sign ofsaid coefficient being opposite in said two intervals.

ll. The coupling device of claim 10 wherein said coupling coefiicientincreases from zero with distance along one of said intervals to apositive maximum, reverses abruptly in sign to a negative maximum, anddecreases with distance along the other of said intervals to zero.

12. The coupling device of claim 10 wherein said coupling coefiicient iszero at a point between said coupling intervals, increases with distancefrom said point to a positive maximum in one interval and increases withdistance from said point to a negative maximum in the other of saidintervals.

13. A coupling device comprising a pair of partially contiguousconductive wave guides of rectangular cross section, means for couplingsaid guides comprising a common wall in said contiguous portion havingelongated aperture means therein, said wall being the wider wall in atleast one of said guides, said aperture means extending at difierentlongitudinal positions along said wide wall through points that includea point on the center line of said wide wall and points displaced onboth sides of said center line.

14. A coupling device comprising a pair of electromagnetic wavetransmission lines, means for coupling said lines with a couplingstrength that varies with distance along a longitudinal interval of saidlines, said lines each having phase constants that are different fromeach other along a substantial portion of said interval, said difierencevarying with distance along said interval inversely to said couplingstrength variation.

15. A coupling device in accordance with claim 14 in which saiddifference and said coupling strength vary at substantially larger ratesin the vicinity of a center portion of said interval than in portions ofsaid interval on either side of said center portion.

16. A coupling device in accordance with claim 15 in which saiddifierence varies as a function of cos and said coupling strength variesas a function of sin 6, wherein 0 is a parameter that increases withdistance along said interval in said center portion at a substantiallylarger rate than in said portion on either side of said center portion.

17. Directional coupling apparatus for electromagnetic wave energycomprising, a conductively bounded wave-guiding channel having a pair ofwide and a pair of narrow walls, a conductive divider extendinglongitudinally along said channel and perpendicularly to said wide wallsof said channel, said conductive divider being spaced nearer to a firstnarrow wall of said channel than to the second narrow wall in at least afirst transverse cross section perpendicular to said wide walls andbeing equidistant between said first and second narrow walls in a secondtransverse cross section perpendicular to said wide walls, saidconductive divider having a coupling aperture extending therethrough toinclude in at least said first transverse cross section a portion of theconductive boundary of said coupling aperture, whereby electromagneticcoupling is provided between the portions of said,

channel on either side of said divider, said aperture having a dimensionin the direction parallel to said narrow walls which varies along alongitudinally extending portion of said channel.

18. Apparatus according to claim 17 wherein said second transverse crosssection also includes a portion of the conductive boundary of saidcoupling aperture.

19. Apparatus according to claim 18 wherein the dimension of saidaperture in the direction parallel to said narrow walls in said firstcross section is smaller than in said second cross section.

20, Apparatus according to claim 19 wherein the dimension of saidaperture in the direction parallel to said narrow walls in said secondcross section is a maximum.

21. Apparatus according to claim 20 wherein the dimension of saidaperture in the direction parallel to said narrow walls in a third crosssection of said divider is zero, said divider being spaced nearer tosaid first narrow wall than to said second narrow wall in said thirdcross section, and wherein said first cross section lies between saidsecond and third cross section.

22. Apparatus according to claim 18 wherein said divider is spacedcloser to said second narrow wall than to said first narrow wall inanother cross section, and wherein said second cross section is disposedbetween said first cross section and said other. cross section, saidother cross section including portions of the conductive boundary ofsaid coupling aperture.

23. Apparatus according to claim 22 wherein the largest dimension ofsaid aperture in a direction parallel to said first and second narrowwalls and perpendicular to said wide walls exists in said second crosssection.

References Cited in the file of this patent UNITED STATES PATENTS

